Apparatus for examining teeth



July 15, 1952 K. N. AXELSSON ET AL 2,603,753

APPARATUS FOR EXAMINING TEETH Filed April 29, 1947 I s 7 5' 2. E

Patented July 15, 1952 2,603,753 7 APPARATUS FOR EXAMJNING TEETH Karl Nils Axelsson and Carl HildingBjiirn,

Djursholm, Sweden Application April 29, 1947; Serial No. 744,616 In Sweden April 29,1946 I 6 Claims. (cram-36.15111.

An important problem in dentistry which has {not hitherto been effectively solved is the determination of the state of teeth as regards the excitability of the sensitive elements. On one hand it is possible by such determination to get an idea of the general state of. the tooth and the tooth pulp and to diagnose pulp diseases, and on the other hand one can estimate the effectof applied anesthesia. The present invention relates a method and means for -minations. 1

making such deter- Systematic investigations have shown that by applying, to the tooth, electrical impulses of a 1 suitable shape by means of an appropriate electrode and by determining the current value atv which these impulses are just being perceived (threshold value) one can obtain an extraordinary good knowledge of the state of the tooth. However, in order. to obtain observations which may easilybe repeated; or compared, it

has proved necessary to use electrical impulses of a particular shape. Thus, the impulses applied to the tooth should be unidirectional and preferably of a substantially rectangular shape. Furthermore, it is of a particular importance that the impulse exhibit a steep wave front, i. e; the current should rise suddenly from zero to the intended value, thereuponremain at this value during a'certain period,- and finally rapidly decline to zero, The requirement that the current should dropv abruptly at the end of the impulse need not be fulfilled with the same degree of accuracy as that governing the rise of .thecurrent at the front sidenof the impulse, however. If

the demand for reproduceable results is small one.

may also usetriangular or other impulses, though with les advantage. The investigationscarried out have also shown that the duration'of the impulse is of a rather great importance. The use of too short impulses is accompanied by marked disadvantages. Thus the range of operation of the device will be narrower since with short im'- pulses larger current are required. to obtainexcitation. Moreover, unavoidable variationsin the duration of the impulse may influence the ac'= curacy of the results. On theother hand too lon impulses are alsoundesirable since damages to the tissues may then be. feared for different reasons. It has been found that. the duration'of the impulse preferably should be from l to 2 00 msec., the most suitable interval being from about 5120 40 msec.

The strength of the impulse, or, current I strength, to some extent depends uponthe electrade sed- W h awn-excitation electrode havinga contact surface area of about -3-mrn.

1.75 x 1.75 mm.) it will be found that a normal healthy tooth exhibits a threshold value of-some microamperes, whereas a tooth having apdiseased pulp, or one which has been anesthetised in some known manner, will react only. atahigher current strength. Should noresponse be obtained at a currentstrength of from 40 150,641

. micro-amperes, this is an indication that;- the pulp is eitherdead or has been ;deprivedof-;its

sensitivitybyanesthesia.

Furthermoreit should be notedjthatthe polarity of the impulse has acertain influence; Thus,

-it has been found preferable to have the current pass from thetooth to the electrode, this giving lower :threshold values than current flowinthe opposite direction. a

e The application of current to the-tooth i most simply effected by having the patient grip ahand electrode connected to the positive pole of an impulsesource while simultaneously contacting the'tooth :by means of an excitation electrode connected to the negative pole of said source.

Taking oif the current from the tooth bymeans of the excitation electrode may present some difficulty. Detailed investigations have shown-that the excitation electrode preferably shouldxconsistof a material which is somewhat yielding :so as to conform itself to the shape'of the-toothand which is also capable of absorbinganelectrolyte serving as a current conductin medium. -;Certain woods, such as aspen, alder, birch, pine, balsa and spruce, which havebeen soaked'with physiological saline solution, for instance, have proved excellent for the purpose. However, the invention is not limitedto .the use of an excitation electrode 7 of these materials, it being possible to usealso electrodes of othertypes, such as electrodes consisting of a conducting-rubber material; iwire bundles etc. 1

For producing the desired electrical impulses any suitable apparatus may be used. In thelfollowing a preferred form of apparatus will be described with reference to the accompanying drawins, wherein: I

Fig. 1 is a circuit/diagram of the apparatus; Fig; 2 is a voltage diagram showing the form of the initial voltage. impulse produced; and

Fig. 3 is a current diagram showing the form of the output current impulse.

The. input terminals designated and 2 are .connected to a source of comparatively'high voltage, such as a rectifier R. A voltagedividerfcomprising'two resistors 3 and 4 "is connecte across saidinput terminals. A "condenser 5 reconnected to the voltage divider and'is charged to a voltage sufiicient to cause break down of a glow dis-charge tube I I through which the condenser i discharged, the object of the voltage divider 3, 4 being to prevent the condenser from being charged to the whole voltage available in case the glow discharge'tube II should fail When the con- 7 denser 5 is discharged through the glow discharge tube 1 I current will flow through a resistor 6 causing a voltage drop across the same. The volt.- age across this resistor 6 will have the form desigated ACBDE in Fig. 2. The maximum voltage drop across the resistor 6 will be. nearly equal to the difference between the break down and extinction voltages of the glow discharge tube. It is seen in Fig. 2 that the voltage rises practically immediately to the maximum value and then decays exponentially. The voltage. impulse thus obtained is applied to the control grid 24 of an amplifier tube 23 which is shown in Fig. 1 as being of the screen-grid type but whichmay also be a triode or-a pentode. For certain'reasons it is preferable to use a tube with a' high plate resistance. In order to produce the desired rectang ular output current impulse the voltage impulse is applied to the control grid 24 through resistors I6 and I! which are connected to the sliding contact of a potentiometer 29 through half-wave asymmetric rectifiers I8 and I9. Since the resistance of these rectifiers is very low in the 'forward direction the voltage-of the control grid 24 can rise only to the value whichis determined by the setting of the potentiometer 23. The impulse applied to the control grid 24 therefore will have the shape ACDE shown in Fig. 2, i. e. it will exhibit a steep front and a somewhat less steep decline which is the shape that investigais preferably connected to the output terminal 32 nearest to the plate 26, whereas the hand electrode is connected to the other output terminal 33. In order to ascertain that the amplifier system function correctly, a glow lamp 2I with a resistor 20 across it is provided which may be connected in the plate circuit of tube 23 by means of a switch .22. At a certain setting of the potentiometer29 corresponding to a voltage drop across the resistor 20 equal to the break down voltage-of the glow lamp 2| this lamp will flash each time an excitation impulse is sent out. A resistor 30 connected in the plate circuit of tube 23 serves to prevent dangerous currents from flowing in case of the tube 23 becoming defective.

In observing the reactions of the patient the operatorrshould preferably be warned when the excitation impulse is sent out. On the other hand it is important that the patient himself be unaware of this momentas otherwise he may be inclined to respond to unduly low current strengths. For the same reason the excitation impulses should be sent out at irregular intervals, or at times determined by the operator. The strength of the impulses is generally varied between 0.1 and 15 microamperes per 1 mm. electrode surface. For indicating when an excitation impulse goes out a glow lamp 9 may be provided. This glow lamp may be connected in parallel with the resistor 6 by means of a switch I0 and is caused to fiash by the voltage appearing across this resistor at each impulse. Since in many cases this voltage may be lower than the break down voltage of the glow lamp 9 the same may be backed by thevoltage drop across a resistor I through which the entire current of the apparatus flows.

tions have shown to be suitable. The voltage impulse applied to the control grid 24 releases a current impulse in the plate circuit 26 of the tube in a known manner, this current impulse being of the same general shape as the impulse ACDE, as

shown in Fig. 3. Sincethe setting of the potentiometer 29 determines the maximum voltage value applied to the control grid 24, which value corresponds to the voltage A-C (Fig. 2), it is possible to vary the strength of theoutput current impulse by adjusting the setting of the potentiometer. In order to obtain more uniform calibration of the potentiometer dial, the amplifier tube 23 may suitably be of the variable-mu type. If'the potentiometer 29 is of the linear type, adjustment of the same through equal angles will always bring about equal percentage changes in the strength of the output current impulse. The screen grid 25 of the amplifier tube 23 is connected in the usual manner to comprising resistors 27 and 28. s

In order to provide for easy calibration of the apparatus a switch comprising a movable arm I5 and two fixed contacts I3 and I4 i provided. When the arm I5 goes to contact I4 the impulses a voltage divider from the glow discharge tube II will pass to the amplifier tube 23 as described. If, on the other hand, arm I5 is moved to contact I3, a voltage corresponding to the setting of the potentiometer 29 is constantly applied to the control grid 24 of the amplifier tube by means of a resistor I2 connected to a positive voltage. The output current strength then obtained corresponds to the maximum current strength of the current impulse and may conveniently be determined by a meter connected in the plate circuit of 26. 1

When using the apparatus above described for 1 tooth examining purposes the excitation electrode A push-button switch 3! is connected in the lead to the glow discharge tube II. This switch is normally open and is closed by the operator when it is desired to produce an impulse. Y

Finally the apparatus may be earthed for A. C.

- by means of a condenser 8.

The form of the discharge curve shown in Fig. 2 is determined by the characteristics of the glow discharge tube II and the values of condenser 5 and resistor 6. The interval C'D depends upon the voltage at which the rectifiers or asymmetric units I8 and I9 begin to operate which in turn is governed by the adjustment of the movable contact of potentiometer 29, The interval between two impulses depends upon the values of condenser 5 and resistors 3 and 4, the characteristics of tube II and the voltage between I and 2.

The output current from terminals 32, 33 depends upon the characteristics of tube 23, the screen grid voltage, i. e. the voltage drop over resistor-28, the adjustment of the movable contact of potentiometer 29 and to some degree upon 'the plate voltage of tube 23, i. e". upon the voltage between land 2 and the value of resistor 30."

In a practical embodiment of the apparatus the voltage between I and 2 was about 800 volts. Tube I I was a type OA4G,- resistor 3 was 4 megohms, condenser 5 was I id resistor 4 was 5 megohms, resistor 6 was 10,000 ohms and the voltage drop over resistor I was about'30 volts. The asymmetric units 'I8and I9 were constituted by the two diode-systems of a type 6H6 tube, the'resistors I6 and I1 (not very critical) were each .5 to 2 megohms. Tube 23 was a 6K7, and the voltage drop over resistor 28 was 45 volts and over potentiometer 29 about 60 volts. g

If the movable contact of thepotentiometerj 29 was placed at about 10 volts from the lower-end.

the plate current of tube 23 (resistor 30=about1 The smaller the resistor 3, the greater'will be A the number of. impulses generated per second,

and the greater the condenser 5, the lesser will be the number of impulses generated per second.

The smaller the resistor ii and the smaller the condenser 5, the more rapid will be the dis charge and the shorter will be the impulse duration, other factors remaining constant. The higher the voltage is between the movable contact of the potentiometer 29 and thelower-end of the potentiometer resistance, the shorter will be the duration of the impulse. I

While we have described our invention in connection with a preferred embodiment of the apparatus, it is to be understood that various modifications may be made without departing from the spirit'of the invention as defined by the appended claims.

What we claim is:

1'. In an apparatus for examining teeth by an electrical excitation method, a generator for producing electrical impulses of a substantially triangular shape and exhibiting a steep front, an amplifier including an amplifier tube, meansincluding a resistance element for applying said triangular impulses to the control grid of said amplifier tube, and a rectifier connected between said control grid and an adjustable voltage source, said rectifier serving to determine the maximum voltage applied to thecontrol grid so as to produce substantially rectangular current impulses in the output circuit of the amplifier tube.

2. An apparatus as claimed in claim 1, wherein the amplifier tube is of the variable-mu type.

3. In an apparatus for examining teeth by an electrical excitation method, a generator for producing electrical impulses of a substantially triangular shape and exhibiting a steep front, said generator comprising a condenser and a glow discharge tube for periodically discharging said V condenser, a resistance element connected in the circuit 01', said glow discharge tube, an amplifier including an amplifier tube, means including a resistance element for conveying said triangular impulses from the first-mentioned resistance element to the control grid of said amplifier tube, and a rectifier connected between said control grid and an adjustable voltage source, said rectifier serving to determine the maximum voltage applied to the control grid so as to produce substantially rectangular current impulses in the output circuit of the amplifier tube.

4. In an apparatus for examining teeth by an electrical excitation method, a generator for producing electrical impulses ofa substantially triangular shape and exhibiting a steep front, said generator comprising a condenser and a glow discharge tube for periodically'discharging said condenser, a resistance element connected in the circuitof said glow discharge tube, an amplifier" including an amplifier tube, means including a resistance element for conveying said triangular impulses'from the first-mentioned resistance element to the control grid of said amplifier tube,

a rectifier connected-between said control grid and an adjustable voltage source, said rectifier serving to determine the maximum voltage applied to the control grid so as to produce substantially rectangular current impulses in the output circuit'of the amplifier tube, and a glow lamp connected across said first-mentioned resistance element and serving to visually indicate the impulses generated.

5. In an apparatus for. examining teeth by an 7 electrical excitation method; a generator for pro-} ducing electrical impulses'of a substantially triangular shape and exhibiting a steep front an amplifier-including an amplifier tube, means including a resistance element for applying said triangular impulses to the control grid of said amplifier tube, a rectifier connected between said control grid and an adjustableyoltage source,

said rectifier serving to determine the .maximum voltage applied to the control grid-so as to;

produce substantiallyrectangular current impulses in the output circuit of the amplifier tube,

and a glow lamp witha parallel resistor connectable in the output circuit of said amplifier tube. 6. An apparatus as claimed in claim 3, wherein means are provided for manually triggering the impulse generator.

KARL NILS. AXELSVSON. .CARL HILDING BJoRN.

REFERENCES CITED 7 The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Page 448 of "Medical Electricity and Roentgen Rays," by'Tousey, published in 1910 by Wm. B. Saunders Co., Philadelphia, Pa., and London, England. I 

